In recent years, there are an increasing number of systems composed of a server and a client devices and each device has a clock. Such a system works on the precondition that each client clock is synchronized with the server clock.
An example of such systems includes a system in which a server holds a content with a fixed usage expiry and distributes the content with the usage expiry to a client. The client is allowed to play back the content on or before the usage expiry based on its clock.
A patent document 1 listed below discloses a technique of authenticating whether a client clock is in synchronism with a server clock and synchronizing the client clock with the server clock if the two clocks disagree.
[Patent Document 1]
JP Patent Application Publication No. 2004-030572